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From Fraser Adams <fraser.ad...@blueyonder.co.uk>
Subject Re: Subject Filtering with Queues
Date Fri, 06 Jun 2014 10:48:32 GMT
On 06/06/14 09:58, Gordon Sim wrote:
> On 06/05/2014 11:27 PM, Chris Wj wrote:
>> It seems that subject filtering with queues is not working in the 
>> current
>> qpid implementations. What is the recommended way to simulate this
>> behavior? I am used to AMQP 0.9.1 and I am very unfamiliar with creating
>> such filtering behavior with QPID. I would like to programmatically
>> establish queues and set it up so that receivers can specify a 
>> key/subject
>> that matches messages. I'm primarily using Python and would like to keep
>> things as cross platform as possible.
>
> With the qpid.messaging API (which I assume is what you are using?), 
> you can specify an address for your receiver in the form 
> <exchange>/<key> and the library will create a queue for you and bind 
> it to the specified exchange with the specified key.
>
> So e.g. creating  a receiver for 'amq.topic/abc' will then receive 
> messages sent to amq.topic with the routing key being abc.
>
> If you want you can control the 'subscription queue' created in more 
> detail by specifying some link options.
>
> E.g. 'amq.topic/abc; {link:{name:my-queue}}'
>
> would mean the queue was given the name my-queue. Further you can 
> specify details of the declare used to create the queue if needed.
>
> E.g. 'amq.topic/abc; {link:{name:my-queue, 
> x-declare:{auto-delete:False, durable:True}}}'
>
> In these examples, the filtering is applied to messages coming through 
> the exchange, before they are enqueued.
>
> If on the other hand you use an address where the 'node' is a queue, 
> i.e. <queue>/<key>, then for receivers the key will not have any 
> effect with the current qpid.messaging. That pattern implies filtering 
> by subject on the messages after the have been enqueued. [It is now 
> supported by the c++ qpid::messaging (when used with qpidd, the c++ 
> broker), but not by the python qpid.messaging equivalent.]
>
>
>
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>

Hi both,
Rob Godfrey is probably better placed to answer this but given Chris's 
original mail where he says "I am used to AMQP 0.9.1" I'm guessing that 
he's actually using a python client with the Java Broker - is that 
correct Chris?

On the subject (pun!!) of "If on the other hand you use an address where 
the 'node' is a queue, i.e. <queue>/<key>" if that's the behaviour that 
is actually required (though I'd personally use the exchange/queue 
approach) I *wonder* if it could be achieved via message selectors. I 
know that selectors were mostly introduced as part of the AMQP 1.0 work 
but I know that Rob has plenty of users using older versions of AMQP and 
I think that his intention is to have the Java broker honour as much as 
possible (he even intends to allow older versions talk to AMQP 1.0 
Management nodes).

So I can't say for sure, but if the Java broker honours message 
selectors for older AMQP versions it *might* be possible to do something 
like

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 
\"qpid.subject='bill'\"}}"

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 
\"qpid.subject='ben'\"}}"

./drain -b localhost -f \
"queue1; {create: receiver, link: {name: test-link, selector: 
\"qpid.subject='tim'\"}}"

./spout -b localhost --content "Hello World" "queue1/tim"


The main gotcha is I think the subject. AMQP 0.10 (not sure about 0.9.1) 
uses qpid.subject but for AMQP 1.0 the subject is in a standard header 
not a user property and I don't think that there's *currently* a way to 
apply a message selector to that, so if you are planning on using 1.0 or 
trying to interoperate between versions there are still some wrinkles 
with this sort of pattern. I've been meaning to look at that but I've 
been tied up on other things.

As I say I'd tend to follow Gordon's suggestion around using the 
exchange/binding/queue pattern, but the queue/selector pattern 
definitely works on recent Qpid versions (I've no idea about performance 
implications between the two approaches, but Gordon's approach is much 
more the common tried and tested pattern to I'd bet that there'd be 
fewer gotchas!)

Regards,
Frase


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